The present relates to phase reference recovery circuits for use in high speed modems with a relatively complex phase/amplitude signal constellations. As is known to those skilled in the art, medium and high speed modems encode data by selectively varying the phase of a transmitted carrier signal with respect to a reference phase. Most popular medium to high speed encoding schemes use some form of differential phase shift key (DPSK) encoding wherein the differential phase shift between two successive baud times encodes the data bits to be transmitted. Therefore, there is no need for an absolute phase reference in order to properly encode data, once communication is established since it is only the relative phase shift between successive baud times that is important to the decoding process.
For modems usable on conventional voice grade telephone lines, or other band limited communication channels, such as leased telephone lines, there are upper limits on the keying rate for phase shift keyed modems. In other words, as the d.phi./dt characteristics of the signal increase, increased bandwidth is needed in order to get the information through the channel.
As is well known to those skilled in the art, higher speed modem signaling schemes have generally adopted a combination of phase and amplitude modulation to encode data. Therefore, in higher speed modem signaling schemes intended for use on bandwidth limited channels, both the relative change in phase and the relative change in amplitude between successive keyings of the transmitter (baud times) are significant in determining the data transmitted.
Several encoding schemes for transmitting data at 9600 bits per second over telephone lines employ 16 point phase/amplitude signal constellations for encoding 4 bits per baud time. As the complexity of the signal constellation increases, the complexity of the receiver circuitry necessary to properly receive and decode the transmitted signal increases.
Most conventional higher speed modem receiver circuits employ the well known phase locked loop to lock on to an appropriate relative phase of the incoming signal in order to compensate for phase jitter in the received signal and to maintain the ability to receive the signal under conditions of relatively low signal to noise ratio. Also, PLL receiver circuits are very useful in offsetting the effects of frequency drift and offset from the specified carrier frequency which are often encountered under practical conditions.
The present invention is particularly directed to modems adopting encoding schemes which have signal constellations for which the phase and amplitude changes between successive baud times are significant for encoding the data and for which phase lock loop circuits can converge on more than one local minimum. A particular species of such a modem is one conforming to CCITT specification V.29. Conventional modems which use such constellations employ a training sequence, such as that specified in CCITT specification V.29, to allow the receiver of each modem to determine whether the phase lock loop circuit has locked onto the correct local minimum in its error signal in order to decode data. This requires a predetermined handshaking and training sequence in which two modems each transmit a predetermined training sequence to allow the receiver of the other modem to cause its PLL circuit to lock onto the correct local minimum. For example, the training sequence specified in V.29 takes approximately 253 milliseconds, or approximately one quarter second.
In operations of such modems, whenever synchronization is lost due to noise on the line, or some other error, the modem which detects loss of synchronization must request a retraining sequence from the modem with which it is communicating. Data communication must stop, and a lengthy retraining sequence, as described above, must be re-executed in order to reestablish proper communication. As will be apparent to those skilled in the art, this conventional arrangement renders modems employing the above described type of signal constellation impractical for fast turnaround half duplex operation. As is known to those skilled in the art, fast turnaround half duplex operation is a mode of operation of a modem in which the modems actually operate in a half duplex mode, i.e., only one modem is transmitting at any given time. The fast turnaround mode of operation is one in which the pair of modems rapidly switch their modes of operation between transmit and receive. In other words, the direction of communication, for example from modem A to modem B, turns around quickly to communication from modem B to modem A in such devices.
The main purpose of fast turnaround modems is to allow high speed digital communication at high bit rates between a pair of modems with the following operational features: (a) the modems, in many applications, appear to be full duplex to the users; and (b) each modem can occupy all of the available useable bandwidth of the communication channel since the system is half duplex.
For modems designed to be used on information channels with relatively narrow available bandwidth, fast turnaround modems can approximate the performance of full duplex modems operating at the same bit rate for many applications. The advantage of using half duplex operation is that simpler and less expensive signal processing circuits can be used in the receiver, and a greater noise immunity is achieved, as compared to high speed full duplex communication schemes, such as that specified in CCITT Recommendation V.32.
As is known to those skilled in the art, fast turnaround high speed modems can mimic the performance of full duplex modems operating at the same bit rate in many applications. For example, if a user at a terminal is working with a database at a remote computer, relatively little information needs to be transmitted from the terminal to the database computer. So long as the modems turn around with sufficient rapidity, the communication system will appear to the user, as a practical matter, to be operating as if it were a pair of 9600 bit per second full duplex modems.
At least one study has provided quantitative data concerning the perception of a terminal user in the circumstance described immediately above. If the criteria are met, communication over a data link connected by two half duplex fast turnaround modems will appear to the interactive terminal user to be the same as full duplex communication. The design criteria in question is the maximum delay between the time the user operates a key at a terminal keyboard and the time that the character is echoed back to appear on the screen of the user's terminal. The study referenced above found that as long as this delay does not exceed 200 milliseconds, the communication link will appear to the user to be full duplex in nature.
As is known to those skilled in the art, most data communication schemes between terminal devices and host computers use character echoing arrangements. In a character echoing communication scheme, the transmission of a character from a terminal device sends the character to the host computer which both receives and processes the character according to the program it is running, and transmits the character back over the data communications link to the terminal. It is receipt of the character back from the host computer at the terminal, which causes the character to appear on the screen. This arrangement has been standard for many years because, inter alia, it allows the terminal user to have some confirmation of the receipt of the data he or she is sending by the host computer.
In most full duplex data communications schemes, the round trip delay between key operation and appearance of the character on the screen is much less than 200 milliseconds.
The above noted study determined that once the delay between key stroke operation and the appearance on the screen of the echoed back character exceeds 200 milliseconds, users begin to detect the intervening delay, and thus the half duplex operation of the system. It will be quickly appreciated that use of conventional training sequences of the type described in CCITT Recommendation V.29 simply cannot meet this criteria. Since the training sequence specified in Recommendation V.29, by itself, exceeds 200 milliseconds, the conventional approach to this type of communication cannot be used to implement a modem meeting the above cited design criteria.
Therefore, there is a need in the art to provide a modem receiver usable in a relatively complex phase/amplitude signal constellation communication scheme, which can avoid the necessity of relatively long training sequences. In particular, it is preferable if such training sequences could be reduced by approximately an order of magnitude.